Higher levels of integration in VLSI circuits have led to exposure patterns of increasingly small linewidth. This has created a need for exposure light sources of shorter wavelength in the lithography systems or steppers used to write circuit patterns on semiconductor wafers. The i-line (wavelength, 365 nm), once the light source of choice in lithography steppers, has been largely supplanted by the KrF excimer laser (248 nm), and today ArF excimer lasers (193 nm) are starting to see industrial use. Also for providing higher NA, the introduction of the immersion lithography is under investigation.
In unison with the development of light sources with shorter wavelength and lenses with increased NA, there exists a need for higher precision not only in the optical components (e.g., lenses, windows, prisms) used in exposure tools, but also in the photomask-forming synthetic quartz glass substrates, known as reticles, serving as the IC circuit pattern master. With respect to the ArF excimer laser, in particular, many important problems remain unsolved including high UV light transmission and high transmission uniformity as is the case with optical components, as well as stability and uniformity of transmission to excimer laser radiation, and even a reduction of in-plane birefringence depending on the future exposure system, for example, use of polarized illumination.
Two processes are commonly used for making synthetic quartz glass ingots from which synthetic quartz glass substrates are made. In a direct process, a silica-forming raw material is flame hydrolyzed, forming fine particles of silica which are melted and deposited concurrently to effect growth. In a soot process, a silica-forming raw material is flame hydrolyzed, forming fine particles of silica which are deposited to effect growth, then later vitrified into clear glass.
In these processes, measures are usually taken to avoid incorporation of metal impurities which can cause ultraviolet absorption. In the direct process, for example, a vapor of a high purity silane or silicone compound, typically silicon tetrachloride is directly introduced into the oxyhydrogen flame. It is subjected to flame hydrolysis to form silica fine particles, which are deposited directly on a rotating heat resistant substrate of quartz glass or the like, where the material is melted and vitrified into transparent synthetic quartz glass.
The transparent synthetic quartz glass prepared in this way exhibits a good transmission even in the short wavelength region down to about 190 nm. It is thus used as transmissive material for ultraviolet laser radiation, specifically i-line, excimer laser beams such as KrF (248 nm), XeCl (308 nm), XeBr (282 nm), XeF (351 nm, 353 nm), and ArF (193 nm), and the 4-fold harmonic of YAG (250 nm).
The most important transmission to UV light is the transmission to the 193.4 nm wavelength light in the case of an ArF excimer laser. The transmission of quartz glass to light at this wavelength region decreases as the content of impurities rises. Typical impurities include alkali metals such as sodium, and other metallic elements such as copper and iron. If the silane or silicone starting material used to produce synthetic quart glass is of very high purity, the concentration of such metallic impurities present within the quartz glass can be brought down to below the level of detection by a high sensitivity detector (<1 ppb). However, because sodium and copper have relatively high coefficients of diffusion into synthetic quartz glass, such impurities of the external origin can often diffuse and admix in during heat treatment. Special care must be taken to avoid such contamination during these treatment operations.
Besides the impurities discussed above, intrinsic defects present in synthetic quartz glass are known to have impact on the transmission. The intrinsic defects are characterized by too much or too little oxygen for the Si—O—Si structure making up the synthetic quartz glass. Well-known examples include oxygen deficient defects (Si—Si, which absorbs at 245 nm) and oxygen surplus defects (Si—O—O—Si, which absorbs at 177 nm). However, such defects, or at least those which are measurable by spectrophotometry, are excluded from synthetic quartz glass for UV application to begin with. Of greater concern are more subtle defects, such as those of excessively stretched or compressed Si—O—Si bonds and those in which the Si—O—Si bond angle falls outside the stability range.
Such subtle defects are said to cause minute absorption in the UV region of wavelength 200 nm or shorter. It is believed that these defects result from some factors involved in the synthetic quartz glass manufacturing process. In the direct process described above, for example, a synthetic quartz glass ingot prepared thereby has a subtle difference in transmission between center and peripheral portions, as analyzed in a plane perpendicular to the growth direction, typically a difference of about 0.5% at the wavelength 193.4 nm of ArF excimer laser. This transmission difference is believed attributable to a temperature distribution in the silica growth/fusion face. It is believed that the peripheral portion assumes a subtle unstable structure due to a lower surface temperature at the peripheral portion than at the central portion and thus has a lower UV transmission.
To remove such unstable structures, JP-A 7-61823 discloses a method in which the growth rate of quartz glass produced by the direct process is held at or below a level of 2 mm per hour. Although this method does appear to work, its very slow growth rate leads to poor productivity and an economical problem.
As effective means for improving the UV transmission of synthetic quartz glass ingots, Japanese Patent No. 2762188 discloses that the absorption of light at wavelength 200 nm or shorter due to the contamination of synthetic quartz glass blocks during heat treatment is eliminated by irradiating UV radiation of wavelength in the range of 150 to 300 nm, desirably 180 to 255 nm.
Like the UV transmission, stability of synthetic quartz glass to excimer laser irradiation is also important. The stability is a very important factor particularly in the case of ArF excimer laser because the ArF excimer laser reportedly causes five times more damage than a KrF excimer laser.
When synthetic quartz glass is irradiated with ArF excimer laser light, there arises a phenomenon that Si—O—Si bonds undergo cleavage by the very intense energy of laser light, forming the paramagnetic defects commonly known as E′ centers which absorb 215 nm light. This brings a loss of transmission at 193.4 nm to synthetic quartz glass. It is also known that another phenomenon, commonly referred to as “laser compaction,” arises that a rearrangement of the network structure of quartz glass increases the glass density.
It is known that reducing the number of intrinsic defects in quartz glass and setting the hydrogen molecule concentration in quartz glass above a certain level are both highly effective for improving the stability of synthetic quartz glass to laser irradiation.
The fact that hydrogen molecules in the quartz glass inhibit damage to the glass by excimer laser irradiation is well-known in the art and has been the subject of active investigation ever since it was revealed in JP-A 1-212247.
With respect to hydrogen molecules, as disclosed in JP-A 7-43891, particularly in an accelerated irradiation test of operating ArF excimer laser at a high energy per pulse level of 100 mJ/cm2, if more hydrogen molecules are present, the absorption at wavelength 193.4 nm increases at the initial irradiation stage, but mitigates during continued irradiation over a long term. Inversely, if less hydrogen molecules are present, the absorption at 193.4 nm is weak at the initial irradiation stage, but increases during continued irradiation over a long term. It is thus necessary to control as appropriate the concentration of hydrogen molecules in synthetic quartz glass.
While the direct process is designed in pursuit of productivity or intended for improved yields, some synthetic quartz glass ingots prepared thereby contain much more hydrogen molecules. This is due to the process conditions where the oxyhydrogen gas balance corresponds to an excess of hydrogen relative to the oxygen stoichiometry. These ingots are thus susceptible to increased initial absorption when irradiated with ArF excimer laser radiation.
There are two ways to include an appropriate level of hydrogen molecules in synthetic quartz glass. One method is by suitably adjusting the ratio of hydrogen, propane and oxygen used as the combustion-supporting gases during growth of a quartz glass ingot for thereby introducing hydrogen molecules into the growing ingot. This approach allows the concentration of hydrogen molecules in the synthetic quartz glass ingot to be adjusted within a range of about 1×1017 to 2×1019 molecules/cm3.
The other method is by heat treating a synthetic quartz glass body within a hydrogen atmosphere, allowing for thermal diffusion of hydrogen molecules from the atmosphere into the glass body. This method has the advantage of possible strict control of the hydrogen molecule concentration. At the same time, it also has a number of significant disadvantages. Specifically, there is a need for a safety measure for avoiding the catastrophic explosion because it uses hydrogen gas. The heat treatment may allow impurities to diffuse into the quartz glass. These problems can be solved by the use of an ordinary safety measure, a high purity carbon material and the like.
The current concern about optical members used with the ArF excimer laser is a change of refractive index by changes of density and birefringence, known as compaction and rarefaction of glass, during laser irradiation at a practical level of energy and a practical number of shots. It is believed that these phenomena depend on the hydroxyl (OH) group concentration in glass. The solution for restraining or avoiding these phenomena is to reduce the OH concentration in glass. The OH concentration largely originates from the manufacture process of synthetic quartz glass. It is known that the manufacture by the soot process is easy to reduce the OH concentration. For this reason, the soot process is employed rather than the direct process.
Of the current most concern in the practical use of ArF excimer laser, for example, are the suppression of initial absorption and the uniformity of structural factors including birefringence upon laser irradiation within the surface of a synthetic quartz glass substrate.